Summary of Work/Abstract Autoantibodies are pivotal for the pathogenesis of many autoimmune diseases such as system lupus erythematosus (SLE). T follicular helper cells (Tfh) cells are important players in both normal immune responses and autoimmune disease via contact dependent and independent mechanisms to provide helping signals to B cells in the germinal centers (GCs). Tfh cells promote GC B cell proliferation and survival, Ig class switch and affinity maturation, and memory B cell and plasma cell formation. However, deregulated Tfh can trigger abnormal GC B and memory B cell responses to produce autoantibodies and contribute to autoimmune diseases. Tregs, particular Tfr, suppress Tfh cells and GC responses and are critical for prevent autoimmune diseases. Abnormal Tfh and Tfr cells have been associated with or are the causal factors of autoimmune diseases in human patients and/or in autoimmune mouse models. However, the origin of pathogenic Tfh cells in autoimmune diseases has been elusive. Diacylglycerol (DAG) is a critical second messenger that relays proximal signal from the TCR to downstream pathways such as RasGRP1-Ras-Erk1/2 and PKC?-IKK-NF?B pathways as well as PI3K-mTOR signaling. DAG kinases (DGKs) ? and ? terminate DAG signal by conversion to phosphatidic acid. Our preliminary studies with Treg-specific DGK? and ? double deficient mice have revealed critical roles of these enzymes for proper function and stability of Tregs to maintain self-tolerance and to prevent autoimmune diseases. Building strong preliminary data, we will test the novel hypotheses that deregulated conversion of Treg/Tfr to exTreg-Tfh cells is a major source of pathogenic self-reactive Tfh cells that trigger deregulated GC and autoantibody responses that lead to autoimmune diseases and that DGK? and ? serve as a checkpoint to brake the conversion. The proposed studies are expected to improve understanding of the mechanisms that control Treg stability and function and to illustrate the origins of pathogenic self-reactive Tfh cells, and provide novel understanding for the pathogenesis and treatment of autoimmune diseases.